The present invention refers to a process for inhibiting the formation of polymer buildup on the inner surfaces of polymerization reactors by coating such surfaces with an alkaline solution of a linear or branched poly(hydroxybenzyl) compound, poly(hydroxybenzylether) compound, or copolymers or mixtures thereof.
In the industrial production of vinyl polymers, a great variety of chemical processes are used. Those which are particularly relevant to our invention include suspension, dispersion, microsuspension and emulsion processes. In those processes, the reaction medium consists mainly of: water, suspension agents, dispersants or emulsifiers (depending on the case) and one or several substituted monomers of ethylene (which are typically insoluble in water).
Depending on how stable the heterogeneous and discontinuous medium is, vigorous shaking or other kind of agitation is needed to keep apart the small entities of monomer where the chemical transformation is to be carried out. Given the instabilities associated with the heterogeneous medium, the imperfections in the homogenizing flux pattern, those of the temperature transfer system, or the irregularities in the topography of the reactor walls, some polymer particles may tend to separate from the bulk of the solution and to adhere to the inner surfaces of the reactor. As this polymer remains adhered during several reaction cycles, it can cross-link (form a polymer of larger molecular weight than the initial one), and serve as a seed for gradual accumulation of more polymeric material.
The adherence of polymeric material to the walls of a reaction vessel, as well as to the stirring accessories (rods, flux-breakers, etc.), can reduce the quality of the polymer being prepared. Such adhered material affects the process in several ways: it reduces thermal conduction between the heat transfer system and the reaction medium, it affects the homogenizing performance of the mixing elements by modifying their effective shapes and sizes, and it alters the optical and mechanical properties of the final product when it peels off the walls and passes to the polymer bulk, since such granules are substantially harder and less porous than the normal polymer. In addition, it is necessary to minimize the generation of the amount of scale since: 1) its removal causes extra down-time and consequently a deceleration of production, and 2) the opening of the reactor for its proper cleaning (mechanical and/or with pressurized water), allows the release of the monomer to the plant surroundings, which hinders maintaining monomer concentration within the limits established by environmental protection and industrial safety regulations.
Several patents have been granted for coating polymerization reactors in order to prevent the accumulation of polymer on their surfaces, among which are U.S. Pat. No. 4,256,864, which describes the application of an aqueous solution of a complex resulting from mixing a hydrophilic uncross-linked polymer (as poly(vinyl alcohol) or hydroxypropyl methyl cellulose) with an inhibitor such as disodium salt of bisphenol A or glucose thiosemicarbazone; U.S. Pat. No. 4,579,758, in which the base substance for the coating is an oligomer obtained from the condensation of benzaldehyde and resorcinol; U.S. Pat. No. 4,622,245, which utilizes as a surface coating the blend of a polymeric compound which contains hydroxyl groups (such as polyvinyl alcohol) and a derivative of salicylic acid; U.S. Pat. No. 4,200,712 which employs as the primary substance a product of the condensation of an alkyl or halogenated phenol; U.S. Pat. No. 4,263,421 where the coating is applied by means of vinyl chloride in vapor form; U.S. Pat. Nos. 4,080,173 and 4,228,130 that relate to self-condensing products of polyhydric phenols; and Canadian Patent 1,181,899 (equivalent to Japanese application JP-54-107991), which deals with the use of phenol-formaldehyde resins with a modifier such as resorcinol, hydroquinone or bisphenol A.
This summary of the state of the art shows the extensive variety of compounds and methods used in the area of polymerization reactor coating. However, new materials and procedures which effectively diminish the formation of scales are disclosed and claimed herein.